WELDING RESEARCH Fig. 11 — UTS, yield strength, and elongation of the base plate and the joints obtained with different conditions. JULY 2016 / WELDING JOURNAL 243-s A B C D and polished applying standard metallographic techniques to provide a cross-sectional view, and subsequently etched using a 2% nital etchant for metallographic study. Microstructural and composition analysis of the weld metal were performed using optical microscopy (OM), electron backscatter diffraction (EBSD), scanning electron microscope (SEM), xray diffraction (XRD), and energy diffraction x-ray spectrum (EDS) analyzers. To test the mechanical properties of the welds, tensile and microhardness tests were performed. The tensile test coupons were cut from the metal sheets using an abrasive waterjet cutting machine. The tensile specimens comprising the welded joints were machined to the required dimensions (Fig. 3) as per ASTM E8M-04 guidelines (Ref. 27). Microhardness testing was carried out by applying a 200-g load and a dwell time of 10 s followed by optical measurements of the resulting indentation. Experimental Results and Analysis Arc and Metal Transfer Process There are three basic metal transfer modes: short-circuiting, globular, and spray (Ref. 28). At low current and low voltage levels, short-circuiting transfer occurs. At a slightly higher voltage level, globular transfer occurs. The drop size is greater than the diameter of the welding wire, and the detachment is mainly controlled by gravity. When the welding current further increases to a higher level that is above a critical value, called the “transition current,” transfer occurs in the form of relatively small drops that are transferred at the rate of hundreds per second. This spray transfer becomes the predominant mechanism. These metal transfer modes show different arc stabilities, weld formation, spatter levels, and so on. Figure 4 shows the arc and a typical globular transfer process under different welding conditions. The first image shows the end of the last transfer cycle, at the moment prior to the droplet detachment, and the last image shows the droplet prior to detachment in the current transfer cycle. When the experiment was carried out without ultrasonic, the detaching force, mainly the gravitational force, was not large enough to balance out the retaining force for rapid detachment, so the droplet size gradually grew, and the diameters of the droplets prior to detachment were Fig. 10 — Schematic of the test sections of element distribution by EDS analysis. Fig. 12 — A — Fracture surface of BM and welding joints; B — GMAW; C — UGMAW; D — PUGMAW.
Welding Journal | July 2016
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